PURPOSE: To obtain a battery back-up device having high reliability by reading a data area for battery back-up BBU which is written previously by a CPU before a main power supply is cut off by the CPU itself after the main power supply is recovered and comparing the read data area with the prescribed expected value.

CONSTITUTION: A CPU 1 writes an optional data to the BBU data area 21 of a memory device MEM 2 in an initial program loading mode and this data is held within the CPU 1 as the expected value. When a main power supply is cut off, the CPU 1 stops its job while the voltage 102 is kept at a normal level and writes the necessary information to the MEM 2. When the main power supply is recovered, the CPU 1 first reads the data on the area 21 to compare it with the expected value and continues its job automatically as it is if the coincidence is obtained from said comparison. While the CPU 1 discontinues the continuous execution of the job done before the break of the main power supply and then carries out again the job in case of the dissidence.